Flu Influenza viruses

There are two main types of influenza virus, Types A and B. The influenza A and B viruses that routinely spread in people (human influenza viruses) are responsible for seasonal flu epidemics each year. Influenza A viruses can be broken down into sub-types depending on the genes that make up the surface proteins. Over the course of a flu season, different types (A & B) and subtypes (influenza A) of influenza circulate and cause illness. Influenza outbreaks have apparently occurred since atleast the Middle Ages. In the elderly, in infants, and in people with chronic diseases, influenza is associated with especially high mortality.